Preparation of cis-poly(1-ethynylpyrene) using (1-Me-indenyl)(PPh3)NiCCPh/methylaluminoxane as...

Journal of Molecular Catalysis A: Chemical 204–205 (2003) 325–332

Preparation ofcis-poly(1-ethynylpyrene) using(1-Me-indenyl)(PPh3)Ni–C≡C–Ph/methylaluminoxane as catalyst

Ernesto Rivera1, Ruiping Wang, Xiao Xia Zhu, Davit Zargarian∗, Richard Giasson2

Département de Chimie, Université de Montréal, C.P. 6128, Succursale Centre-Ville, Montreal, QC, Canada H3C 3J7

Received 12 September 2002; received in revised form 10 December 2002; accepted 16 December 2002

Dedicated to Prof. Renato Ugo on the occasion of his 65th birthday

Abstract

A combination of the complex (1-Me-indenyl)(PPh3)Ni–C≡C–Ph and methylaluminoxane (MAO) catalyzes the polymer-ization of 1-ethynylpyrene to fairly solublecis-poly(1-ethynylpyrene). The polymers have been characterized by IR, solid state13C and1H NMR, and UV-Vis (λmaxat 346 nm) spectroscopy, and studied by thermogravimetric analysis (T10 ∼ 285–392◦C),differential scanning calorimetry, and size exclusion chromatography (Mw ∼ 103–104 Da; Mw/Mn ∼ 2). The thermal andoptical properties of thesecis-polymers have been compared to theirtrans-analogues prepared using different catalysts. Thedegree of conjugation in thecis-polymers was found to be lower than theirtrans-polymers, presumably due to the twistingand coiling of the main chain.© 2003 Elsevier Science B.V. All rights reserved.

Keywords: Idenyl nickel complex;cis-Poly(1-ethynylpyrene); Polyacetylenes; Conjugated polymers; Insertion mechanism

1. Introduction

Polyacetylene has been the subject of numerousinvestigations since the discovery by MacDiarmid,Shirakawa and Heeger in 1976 that this polymer be-comes highly conducting upon doping[1–3]. In spiteof its considerable promise in the area of electronicmaterials, however, the practical applications of poly-acetylene have remained limited due mainly to its lowsolubility and thermal instability. Fortunately, exten-

∗ Corresponding author. Tel.:+1-514-343-2247;fax: +1-514-343-7586.E-mail addresses: [email protected] (D. Zargarian),[email protected] (R. Giasson).

1 Present address: Instituto de Investigaciones en MaterialesUNAM, Circuito Exterior, Ciudad Universitaria, C.P. 04510, Mex-ico, DF Mexico.

2 Co-corresponding author.

sive research in this field has led to the synthesis of agreat number of thermally stable and processible poly-acetylene derivatives bearing different substituents[4–8]. The latter are highly conjugated polymerswhich might find applications in the construction ofelectronic devices such as light emitting diodes, pho-tovoltaic cells, and non-linear optical systems[9–18].

The optoelectronic characteristics of substitutedpolyacetylenes are highly variable as a function ofboth the configuration of the main chain and its degreeof extended conjugation[19]. These properties are,in turn, determined by the electronic and steric na-ture of the alkyne substituents and the polymerizationmethod. As a result, a great deal of research effort hasbeen devoted to the development of new methods forthe preparation of variously substituted polyacetylenes[20]. Among these, poly(phenylacetylene) (PPA) hasbeen prepared using a variety of catalysts[21] and

1381-1169/$ – see front matter © 2003 Elsevier Science B.V. All rights reserved.doi:10.1016/S1381-1169(03)00314-5

326 E. Rivera et al. / Journal of Molecular Catalysis A: Chemical 204–205 (2003) 325–332

used as a model for configurational and conforma-tional analysis[22–24]. For instance,trans-rich PPAis obtained via metathesis-type polymerization pro-cesses using WCl6 as catalyst in combination with va-rious cocatalysts[25], while Rh catalysts givecis-richPPA via insertion-type polymerization processes[21]. We have also reported the synthesis ofcis-PPAby using systems consisting of the complexes (1-Me-indenyl)Ni(PR3)(X) (R = Ph, Cy; X= Cl, C≡C–Ph) and poly(methylaluminoxane) as cocatalyst; thepolymerization reactions promoted by these systemsalso proceed by an insertion-type mechanism[26].

As part of a research program aimed at the prepa-ration of molecular electronic devices[27,28], weare interested in the synthesis of polymeric materialscontaining pendant pyrenyl groups. The synthesis ofpoly(1-ethynylpyrene) (PEP) via the polymerizationof 1-ethynylpyrene using W, Mo and Rh catalystswas reported recently by Masuda and co-workers[29]. In our hands, good yields oftrans-PEP (Mw ∼2 × 104 to 5× 104; Mw/Mn ∼ 5–10) were obtainedwhen WCl6 was used as catalyst, with or without

Fig. 1. Polymerization of 1-ethynylpyrene with W- and Ni-based catalytic systems.

cocatalysts (Fig. 1), while insolublecis-PEP was ob-tained in low yields with Mo- and Rh-based catalysts[30]. This prompted us to examine the polymeriza-tion of 1-ethynylpyrene using the catalytic system(1-Me-indenyl)Ni(PPh3)(C≡C–Ph)/MAO with a viewto obtaining solublecis-PEP. The present report des-cribes the synthesis of moderately solublecis-PEP(Fig. 1) possessing relatively low polydispersities;these polymers have been fully characterized and theirproperties compared to those oftrans-PEP reportedpreviously[30].

2. Experimental

The polymerizations were carried out under aninert atmosphere using moisture- and oxygen-freesolvents. The preparation of 1-ethynylpyrene[30]and (1-Me-indenyl)Ni(PPh3)(C≡C–Ph) [26] havebeen reported previously; all other reagents used inthese studies, including MAO, were purchased fromAldrich and used as received.

E. Rivera et al. / Journal of Molecular Catalysis A: Chemical 204–205 (2003) 325–332 327

Solid state1H and13C NMR spectra of the polymerswere recorded on a Bruker ARX-700 narrow-borespectrometer. Fast magic angle spinning (MAS) at30 kHz was used. The1H NMR spectra were obtainedby the use of a double resonance MAS probe sup-porting rotors of outer diameter 2.5 mm operating ata 1H Larmor frequency of 700 MHz; the relaxationdelays were adjusted to allow complete relaxation ofthe spins. The IR spectra were recorded on a NicoletFTIR 5 DXB spectrometer using KBr pellets of thepolymer samples.

The molecular weights of the polymers were de-termined by GPC analysis using a Waters 510 liquidchromatograph equipped with�-Styragel columns(THF as eluent) and a Waters 410 refractive in-dex detector (relative to polystyrene standards). Thethermal properties of the polymers were studied bymeasuring the following parameters: 10% weightloss temperature (T10), glass transition temperature(Tg), and melting points (Tm). The thermogravimet-ric analyses were conducted on a Hi-Res TGA 2950instrument (0–1000◦C) and the differential scan-ning calorimetry was carried out on a DSC 2910instrument (−40 to 400◦C) with a heating rate of20◦C/min.

The absorption spectra of the polymers wererecorded using a Varian Cary 1 Bio-UV-Vis spec-

Table 1Conditions for polymerization of 1-ethynylpyrene and properties of the resulting polymers

Run Polymer Ratio Solvent Time (h) Temperature (◦C) Mwa (103 g/mol) Mw/Mn

a T10 (◦C) Tm (◦C) Tg (◦C)

1 Ni-1 1:10:60b Toluene 2 25 2.2 1.932 Ni-2 1:10:60b Toluene 24 25 5.5 1.933 Ni-3 1:10:60b Toluene 48 25 7.2 2.4 392 – 1694 Ni-4 1:10:60b Toluene 24 50 5.2 2.095 Ni-5 1:10:60b THF 24 25 19.1 1.746 Ni-6 1:10:60b THF 24 50 14.2 1.25 335 – –7 Ni-7 1:10:50b THF 48 25 24 2.15 285 – –8 Ni-8 1:10:100b THF 19 25 14.1 2.669 Ni-9 1:10:100b THF 48 25 22.3 1.72

10 Ni-10 1:10:120b THF 72 25 20.8 1.7311 Ni-11 1:10:150b THF 48 25 18.2 2.1912 W-1 WCl6c Toluene 24 25 24.0 2.88 381 330 –13 W-2 WCl6/SnPh4d Toluene 24 25 27.6 4.49 383 345 –14 W-3 WCl6/SnBu4

d Toluene 24 25 277.2 10.67 385 330 –15 W-4 WCl6/BiPh3

d Toluene 24 25 470.5 1.90 389 327 –

a Methanol-insoluble fraction; measured by SEC (THF, polystyrene standard).b Ratio of (1-Me-indenyl)Ni(PPh3)(C≡C–Ph)/MAO/monomer.c Ratio of WCl6/monomer= 1:40.d Ratio of WCl6/cocatalyst/monomer= 1:2:40.

trophotometer (model 8452A) and 1 cm quartz cells.The sample solutions were kept in the Beer–Lambertconcentration regime (1× 10−5 to 3× 10−5 M), andthe optical transparency of the solvent (THF, Aldrichspectrophotometric grade) in the region of interestwas satisfactory.

2.1. Polymerization of 1-ethynylpyrene

The following general procedure was used for thepolymerization of 1-ethynylpyrene with the Ni/MAOsystem; the specific reaction conditions for a se-lected number of experiments are given inTable 1.A solution of 1-ethynylpyrene in toluene or THF(5 mL) was transferred into a Schlenk tube containing(1-Me-indenyl)Ni(PPh3)(C≡C–Ph). After completedissolution, a solution of MAO in the same sol-vent was added; a 1:10 ratio of Ni:MAO was usedthroughout. The reaction mixture was stirred underargon at room temperature or at 50◦C for the desiredtime. The final mixture was treated with acetic acid(five drops) and transferred into methanol (50 mL) inorder to precipitate the resultingcis-PEP as a darkbrown powder. The methanol-insoluble polymer wasisolated by filtration, washed with methanol, anddried under vacuum. The following are the analyticaldata for the polymer sample Ni-9. IR (KBr):ν 3036


(=C–H stretching), 1598 (C==C stretching), 1582,1487, 1414 (C–H bending), 1176, 838 (C=C–H out ofplane bending), 752, 717 and 679 cm−1. Solid state1H NMR (700 MHz):δ 7 (broad, aromatic and vinylicprotons). Solid state13C NMR (175 MHz):δ 137 and125 (broad, all sp2 carbons). UV (THF)λmax: 346 and457 nm. Elemental analysis, calculated for (C18H10)n:C, 95.54; H, 4.45. Found: C, 95.12; H, 4.33.

3. Results and discussion

The catalytic system consisting of (1-Me-indenyl)-Ni(PPh3)(C≡C–Ph) and MAO as cocatalyst hasproven effective for the polymerization of phenyl-acetylene tocis-PPA [26]. This same system reactedwith 1-ethynylpyrene to give dark brown powders(20–80% isolated yields) which were identified ascis-PEP by comparing their physical properties andspectroscopic characteristics to those oftrans-PEPobtained from the polymerization of 1-ethynylpyreneusing WCl6 as catalyst[29,30]. For instance, thesamples of PEP synthesized using the Ni/MAO sys-tem are dark brown powders, which are solublein THF and o-dichlorobenzene, partially soluble inchloroform, and sparingly soluble in toluene. Forcomparison,trans-PEP obtained from catalytic sys-tems based on W[30] are dark purple solids, whichare totally soluble ino-dichorobenzene and THF andonly partially soluble in CHCl3 and toluene; in con-trast, the PEP samples obtained from polymerizationsusing {Rh(nbd)(�-Cl)}2 are practically insoluble inevery organic solvent[29]. Tabata et al. have reportedthat polymerization of�-ethynylnaphthalene using{Rh(nbd)(�-Cl)}2 also gives insoluble polyalkynes;these authors conclude on the basis of Raman stud-ies that these polymers possess acis-cisoidal ge-ometry [31]. On the other hand, polymerizationof phenylacetylene with our Ni/MAO system givecis-transoidal PPA[26]. On the basis of these prece-dents, we assign acis-transoidal geometry to the PEPsamples obtained from the Ni/MAO system.

Section 3.1 gives a summary of the experi-ments aimed at optimizing the polymerization of1-ethynylpyrene with our Ni system; the followingsection describes the spectroscopic characterizationof the polymers, their thermal properties, and a mode-ling study on their conformations.

3.1. Optimization experiments

The polymeric products resulting from the reac-tion of 1-ethynylpyrene with the catalytic system(1-Me-indenyl)Ni(PPh3)(C≡C–Ph)/MAO possessmolecular weights in the range of 103–104 dependingon the polymerization conditions (Table 1). Longerreaction times (24–48 h) were necessary for attaininghigher molecular weights (runs 1–3 in toluene andruns 5 and 7 in THF). The nature of reaction solventplayed an even more important role in determiningthe molecular weight, with THF giving generallyhigherMw than toluene. This may be due to the lowersolubility of cis-PEP in toluene, which would causethe growing polymer to precipitate out of the solutionthereby preventing the chain growth process.

The reaction temperature was an important factorfor the polymerizations carried out in THF: carryingout the polymerization at ambient temperature for agiven reaction time resulted in a diminished monomerconversion (20–50% yields) but gave longer chains;on the other hand, increasing the reaction temperaturefrom 25 to 50◦C resulted in an increase in the yieldof the polymer (80%) but gave shorter chains (run 5versus 6). One possible explanation for this observa-tion is related to the kinetics of the chain transfer step,as follows. It is conceivable that higher temperaturesaccelerate the transfer of the growing polymer chainfrom the Ni center to Al centers in MAO, thereby stop-ping the further growth of the chain; the free Ni cen-ters can then react with more monomers to resume thepolymerization. This sequence would result in a highermonomer-to-polymer conversion but shorter polymerchains.

A number of polymerizations were carried out at25◦C using different catalyst: monomer ratios (runs7–11) in order to probe the importance of this para-meter. The results of these experiments appear to in-dicate that polymers possessing the highestMw valuesare obtained with catalyst: monomer ratios of 1:50 to1:100.

In summary, polymerization of 1-ethynylpyrenewith the catalytic system (1-Me-indenyl)Ni(PPh3)C≡C–Ph/MAO allows the preparation ofcis-PEP withMw ranging from 7.2 to 24× 103 g/mol and polydis-persities between 1.25 and 2.4. In contrast, polymer-ization of the same monomer with WCl6 leads to theformation of trans-PEP with Mw ranging from 2.4


to 47× 104 g/mol and polydispersities between 1.9and 10[30]. A representative sample of the resultsfrom the W-catalyzed polymerizations are included inTable 1for comparison purposes. The physical prop-erties of these materials are discussed inSection 3.2.

3.2. Characterization of PEP

IR, 1H, 13C NMR and UV-Vis spectroscopy wereused to characterize the PEP obtained from thepresent system. The absence in the IR spectra ofthe characteristic alkyne bands (H–C≡ stretching at3296 cm−1, ν(C≡C) at 2000 cm−1, and C–H bend-ing at 700 cm−1) confirm the absence of unreactedalkyne moiety in the polymers. The strong bandsat 3000 cm−1 (=C–H stretching), 1583 cm−1 (C=Cstretching), and 1414 cm−1 (C=C–H bending) are as-sociated with the main chain. The most characteristicabsorption of the pyrene substituent is the=C–H outof plane bending at 839 cm−1.

The solid state1H NMR spectrum ofcis-PEP dis-plays a broad band centered around 7 ppm; the signalsbelonging to the vinylic protons present in the polymerbackbone are masked by those of the aromatic pro-tons. This broadening of the signals was also observedfor trans-PEP obtained from the W-based systems[30] and can be attributed to two phenomena: (a) the

Fig. 3. UV absorption spectra ofcis- and trans-PEP in THF.

Fig. 2. Solid state CP/MAS13C NMR 175 MHz spectra of thepolymers recorded at a spinning frequency of 30 kHz: (A)cis-PEP,(B) trans-PEP.

presence of unpaired radicals arising from the ruptureof some C=C bonds; and (b) the slow motion of therigid polymer chain. The solid state13C NMR spec-trum of cis-PEP (Fig. 2) obtained with the Ni/MAOsystem also exhibits signals at 135 and 125 ppm


corresponding to all sp2 carbons; these signals aresimilar to those displayed by thetrans-polymers.

3.2.1. Thermal and optical properties ofthe polymers

The thermal properties ofcis-PEP were measuredby thermogravimetric analysis (TGA) and differen-tial scanning calorimetry (DSC). The thermal stability(T10), glass transition temperatures (Tg), and meltingpoints (Tm) of these polymers are compared with thoseobtained fortrans-PEP[30] and are shown inTable 1;the following differences in the thermal characteristicsof the cis- and trans-polymers are noteworthy. First,only one sample among the polymers studied showeda glass transition temperature (cis-PEP, Ni-3,Tg =169◦C). Second, alltrans-PEP samples show a melt-

Fig. 4. Conformation of the polymers obtained by molecular modeling: (A)trans-PEP, (B)cis-PEP.

ing or softening point (Tm) between 327 and 345◦C,whereas none of thecis-PEP samples melted or soft-ened in the range studied (−40 to 400◦C). Moreover,in almost all cases, thetrans-polymers are thermallymore stable than theircis-counterparts. For instance,the samples W-1 and Ni-7 have similar molecularweights (ca. 2.66 × 103 g/mol) but theirT10 valuesdiffer by more than 100◦C.

Another noteworthy difference between thecis- andtrans-PEP polymers is that the thermal stabilities oftrans-PEP do not seem to vary much with the molec-ular weight of the sample (T10 around 381–389◦C),whereas thecis-polymers having longer chains arethermally less stable (e.g.T10 for Ni-7 of Mw = 24×103 g/mol is 285◦C versus 392◦C for Ni-3 of Mw =7.2 × 103 g/mol). We speculate that the underlying


reason for the lower thermal stability of thecis-polymers is that thecis-geometry allows a rela-tively facile extrusion of arylacetylene units from thepolymer backbone at elevated temperatures.

The UV-Vis spectra ofcis- and trans-PEP werestudied in order to compare the pattern of electronictransitions in these polymers.Fig. 3 displays thespectra obtained for the polymer sample Ni-7 andits trans-analogue having similar molecular weight,W-1. Both polymers show an absorption maximum at346 nm corresponding to the absorption of the pyrenemoieties in the non-associated state. Thecis-PEP alsodisplays a broad shoulder at 453 nm bearing a tail at537 nm, with a cut-off wavelength reaching 657 nm,whereas thetrans-PEP shows only one broad bandcentered at 565 nm with a cut-off wavelength reaching800 nm. Steady state and time-resolved fluorescencestudies have confirmed that the broad band at 453 nmhas its origin in a ground state complex resulting fromintramolecular interactions between adjacent pyreneunits present in the polymer[31]. The tail around537 nm is likely caused by the short sequences ofconjugatedcis-double bonds present in the backboneof cis-PEP, in contrast to the longer sequences oftrans-double bonds present intrans-PEP, which givesa band at a higher wavelength[30].

The above spectroscopic studies were comple-mented by molecular mechanics calculations (MM2)performed on short segments oftrans- and cis-PEP;the optimized geometries obtained are represented inFig. 4. According to these modeling studies, the mainchain of trans-PEP should exist in an almost planarconformation with internal stacking of the pendantpyrenyl groups (Fig. 4A). The distance between twopyrene units varies from 3.1 (near the backbone) to4.1 Å (far away from it) in this conformation, andthe adjacent pyrene units are almost parallel. Thus,the main chain oftrans-PEP is fully conjugated, con-sistent with the low energy band at 565 nm in theabsorption spectra of this polymer.

The modeling studies also show that it is more dif-ficult for thecis-PEP backbone to adopt a planar con-formation because of the steric interactions betweenpendant pyrenyl groups. This causes a twisting of thechain and a coiling of the polymer backbone that mayfavor intramolecular interactions between pyrene unitsat long distances and in a non-parallel way (Fig. 4B).Distances between pyrene units vary from 4.4 to 5.3 Å

in this conformation. The coiling of the polymer back-bone also decreases the degree of conjugation in themain chain, consistent with the low energy band in theabsorption spectrum ofcis-PEP.

4. Conclusions

Cis-poly(1-ethynylpyrene) with molecular weightsin the range of 7× 103 to 24× 103 g/mol were syn-thesized using the (1-Me-indenyl)Ni(PPh3)C≡C–Ph/MAO catalytic system. The cis-PEP obtainedin this fashion was fairly soluble in THF ando-dichlorobenzene, and exhibited lower thermal sta-bility than trans-PEP prepared with a catalytic sys-tem based on WCl6. The lowest energy absorptionand cut-off wavelengths reached 537 and 657 nm forcis-PEP compared to 565 and 800 nm fortrans-PEP.This is attributed to a lower degree of conjugation incis-PEP due to a coiling of the chain. The band at457 nm proves the existence of a complex resultingfrom intramolecular interactions between neighborpyrene units present in thecis-PEP polymer.

Acknowledgements

The authors are grateful to FCAR and NSERC forresearch support and to Prof. G. Durocher and Dr.M. Belletete for help with the optical characterizationof the polymers. E.R. and R.W. are grateful to theDGAPA of the Autonomous National University ofMexico and FCAR, respectively, for graduate schol-arships.

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Preparation of cis-poly(1-ethynylpyrene) using (1-Me-indenyl)(PPh3)NiCCPh/methylaluminoxane as...

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